By way of background to the present invention, all surfaces above absolute zero emit infrared (IR) radiation into their surroundings. The surface IR emissions from an object result in a detectable contrast and hence the object will have a characteristic thermal image, termed its thermal signature. This phenomenon has found widespread use in military applications such as thermally targeted weaponry and imaging systems for use as soldier aids (night vision goggles (NVG)), and in civil applications such as surveillance of the thermal appearance of industrial processes and equipment in order to recognise deficiencies and hazards. Accordingly, there is a need to simulate thermal signatures of objects using thermal targets in order to train personnel in object recognition and assessment. Said thermal targets must be capable of presenting the correct thermal signature when viewed through equipment such as night vision goggles, IR weapon sights or thermal imaging cameras.
An effective way of providing a thermal signature of an object is to use an electrically heated thermal target. By way of example, U.S. Pat. No. 4,546,983, U.S. Pat. No. 4,659,089, U.S. Pat. No. 4,792,142, and U.S. Pat. No. 5,066,019 describe a variety of conventional electrically heated thermal targets.
It is desirable that the infrared radiation emitted by a thermal target correspond closely with that characteristically emitted by the object being simulated in respect of both the intensity and spatial pattern of the emitted infrared radiation. Typically, the thermal signature of an object is composed of a number of key elements, known as thermal signature cues. Said thermal signature cues enable trained personnel to detect objects from thermal images thereof and to ascertain information about the object under surveillance. Hence, faithfully recreating the thermal signature of an object may require a thermal target having many individual elements of different aspect ratios, sizes and surface temperatures acting as a whole.
The characteristics of the infrared radiation emitted by an electrically heated thermal target are traditionally determined by the thermal and electrical characteristics of the target, which are in turn dependent upon its construction. Electrically heated thermal targets operate by passing an electrical current through resistive heating elements there-within to cause Joulean heating. The heated elements in turn give rise to emission of thermal infrared radiation from surface of the target. The production of heat within the target is a function of the current (and therefore the applied voltage) and the resistivity of the material of which the heating elements are comprised, the latter being dependent on the composition of the resistive material from which the heating element is fabricated and the width and thickness thereof. The amount of IR energy radiated from the heated surface of the target, compared to that expected from its physical temperature, is determined by the emissivity of the surface. The emissivity is generally low for metals and high for polymer materials.
A conventional thermal target may comprise elements which can be modified in a number of ways so as to emit thermal signature cues having desired characteristics.
For example, in U.S. Pat. No. 4,546,983 the intensity of infrared radiation emitted by heating elements within the target may be altered by varying the input voltage to heating elements within the target, or increasing/decreasing the thickness of the electrically resistive layer (thereby modifying the current passing through said heating element). Alternatively, or in addition, the resistivity of the electrically resistive layer may be varied by altering the composition of the resistive layer. In practice, this may be done using mixtures of materials with different bulk resistance.
Another method described in U.S. Pat. No. 4,546,983 for altering the intensity of emitted infrared radiation comprises perforating the resistive layer so as to decrease the area available to generate thermal infrared radiation. The reduction in the intensity of emitted infrared radiation is proportional to the area of the perforations and not due to electro-thermal effects (the current density in the remaining portions of the resistive layer remains unchanged).
Notwithstanding the efficacy of the abovementioned techniques, traditional methods of varying the infrared radiation emitted by a thermal target are difficult to implement during manufacturing and are therefore expensive.
Accordingly, it is an object of the invention to provide a thermally emissive apparatus which mitigates at least some of the disadvantages of the conventional devices described above.